Selective ERα activation disrupts sex organ differentiation and induces expression of vitellogenin II and very low- density apolipoprotein II in Japanese quail embryos

1 The Japanese quail (Coturnix japonica) is a widely-used model species for studying the roles 2 of steroid hormones in avian sex differentiation. The aim of the present study was to elucidate 3 the significance of estrogen receptors alpha and beta (ER and ER) in normal sex 4 differentiation of the reproductive organs in the Japanese quail and in xenoestrogen-induced 5 disruption of reproductive-organ differentiation. Real-time PCR indicated that ER mRNA is 6 expressed in both right and left gonads and Müllerian ducts in both sexes during early 7 morphological differentiation. ER-transcripts were also detected in gonads and Müllerian 8 ducts, but at very low levels. Both receptor subtypes were expressed in the liver and may 9 therefore mediate the expression of estrogen-regulated egg-yolk proteins. Aromatase mRNA 10 was expressed at much higher levels in female than male gonads as early as embryonic day 5, 11 indicating early sex differences in estrogen synthesis. Treatment with the ER-selective 12 agonist, propyl pyrazole triol (PPT), showed that frequently-reported xenoestrogen effects 13 such as ovotestis formation, abnormal Müllerian duct development, and hepatic expression of 14 egg-yolk proteins were induced by selective activation of ER. Taken together, our results 15 suggest that activation of ER is crucial for estrogen-dependent sex differentiation of the 16 reproductive organs and that ER mediates xenoestrogen-induced toxicity during 17 reproductive development in birds.